Conventionally, various kinds of underwater pumps have been developed for the above purpose including the pump which is disclosed in Japanese Patent Publication SHO58-50531.
In these pumps, however, the portion of the rotating shaft where the rotating head such as the agitator is mounted is totally exposed to the water so that any flexible pieces such as cloth or thin plastic strips contained in the sand tend to adhere to or are wound around the exposed rotating portion. These flexible pieces eventually narrow or clog the passage through which the excavated sand flows into the impeller casing so that excavating efficiency is greatly damaged. Furthermore, in an extreme case, such flexible piece may cause the stoppage of the rotation of the agitator and the malfunction of the underwater pump.